Polyesters based on diols and aromatic diacids, often called aromatic polyesters, are used in many industrial applications due to their low cost of production, easy processing, good barrier properties, and strong thermal and mechanical performances. Poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT), as well as PET modified with less than 50 mol % of 1,4-cyclohexanedimethanol (CHDM or, as polymerized, 1,4-cyclohexylenedimethylene) (PETG) and PCT modified with less than 50 mol % ethylene glycol (PCTG) are examples of such polyesters. Amorphous versus semicrystalline morphology, glass transition temperature, crystallization temperature, melting temperature, melt stability heat distortion temperature, tensile and flexural strength, tensile and flexural moduli, and extension to break, are examples of important properties.
Copolymers of 4,4′-biphenyl dicarboxylic acid and 3,4′-biphenyl dicarboxylic acid with a diol such as ethylene glycol are known from U.S. Pat. No. 5,138,022 and WO 2015/112252. A semicrystalline copolyester is obtained when the 4,4′-biphenyl dicarboxylic acid content is 50 mole percent or more. These semicrystalline copolyesters usually have lower glass transition temperatures than desired and/or poor tensile properties such as toughness for particular applications, and in addition have melting temperatures higher than desired for processing. When more 4,4′-biphenyl dicarboxylic acid is incorporated to improve tensile or other properties, the melting temperature is further increased.
The amorphous copolyesters of 4,4′-biphenyl dicarboxylic acid and 3,4′-biphenyl dicarboxylic acid with ethylene glycol generally incorporate more of the 3,4′-biphenyl dicarboxylic acid isomer, and can have undesirably low glass transition temperatures and/or poor tensile properties such as toughness. When more 4,4′-biphenyl dicarboxylic acid is incorporated in an effort to elevate the glass transition temperature or improve other properties, the copolyester becomes semicrystalline.
The industry thus has one or more of the following needs: to improve control over the morphology of the 4,4′-biphenyl dicarboxylic acid and 3,4′-biphenyl dicarboxylic acid copolyesters and or improve the properties of the copolyester due to an increase in the amount of 4,4′-biphenyl dicarboxylic acid that can be used in the amorphous copolyester; to lower the melting temperature of the semicrystalline copolyesters; to increase the glass transition temperature of the amorphous or semicrystalline copolyesters; and/or to improve the tensile or other properties of the amorphous or semicrystalline copolyesters.